Internal combustion engines may include a cylinder block and at least one cylinder head which are connected to one another at an assembly end side to form the cylinders. To control the charge exchange, an internal combustion engine may utilize control elements—generally in the form of lifting valves—and actuating devices for actuating these control elements. Each lifting valve moves, so as to realize, that is to say perform, a valve lift, between an open position and a closed position, and in so doing, during an opening duration, opens up the opening associated with the valve. The valve actuating mechanism for the movement of the valve, including the valve itself, is referred to as the valve drive. The cylinder head often serves to accommodate the valve drives.
During the charge exchange, the combustion gases are discharged via the outlet openings of the cylinders and the charging with charge air takes place via the inlet openings. The valve drives open and close the inlet and outlet openings at the correct times, with a fast opening of the largest possible flow cross sections generally being sought in order to keep the throttling losses in the inflowing and outflowing gas flows low and in order to ensure the best possible charging of the cylinders and an effective discharge of the exhaust gases. The cylinders are also often provided with two or more inlet and outlet openings.
The intake lines which lead to the inlet openings, and the exhaust lines which adjoin the outlet openings, may be at least partially integrated in the cylinder head. The exhaust lines of the cylinders generally merge to form one common overall exhaust line, or else merge in groups to form two or more overall exhaust lines. The merging of exhaust lines to form an overall exhaust line is referred to in general and within the context of the present disclosure as an exhaust manifold, with that part of the overall exhaust line which lies upstream of a turbine arranged in the overall exhaust line being considered according to the disclosure as belonging to the exhaust manifold.
Downstream of the manifold, the exhaust gases may be supplied, for the purpose of supercharging of the internal combustion engine, to the turbines of at least two exhaust-gas turbochargers and if appropriate to one or more systems for exhaust-gas aftertreatment.
The advantages of an exhaust-gas turbocharger for example in relation to a mechanical charger are that no mechanical connection for transmitting power exists or is required between the charger and internal combustion engine. While a mechanical charger extracts the energy required for driving it entirely from the internal combustion engine, and thereby reduces the output power and consequently adversely affects the efficiency, the exhaust-gas turbocharger utilizes the exhaust-gas energy of the hot exhaust gases.
An exhaust-gas turbocharger comprises a compressor and a turbine which are arranged on the same shaft. The hot exhaust-gas flow is fed to the turbine and expands in the turbine with a release of energy, as a result of which the shaft is set in rotation. The energy supplied by the exhaust-gas flow to the turbine and ultimately to the shaft is used for driving the compressor which is likewise arranged on the shaft. The compressor delivers and compresses the charge air supplied to it, as a result of which supercharging of the at least two cylinders is obtained. A charge-air cooling arrangement may be provided, by which the compressed charge air is cooled before it enters the cylinders.
Supercharging serves primarily to increase the power of the internal combustion engine. Here, the air for the combustion process is compressed, as a result of which a greater air mass can be supplied to each cylinder per working cycle. In this way, the fuel mass and therefore the mean pressure may be increased. Supercharging is a suitable mechanism for increasing the power of an internal combustion engine while maintaining an unchanged swept volume, or for reducing the swept volume while maintaining the same power. In any case, supercharging leads to an increase in volumetric power output and a more expedient power-to-weight ratio. If the swept volume is reduced, it is thus possible to shift the load collective toward higher loads, at which the specific fuel consumption is lower. By supercharging in combination with suitable transmission configurations, it is also possible to realize so-called downspeeding, with which it is likewise possible to achieve a lower specific fuel consumption. Supercharging consequently assists in the constant efforts in the development of internal combustion engines to minimize fuel consumption, that is to say to improve the efficiency of the internal combustion engine.
It is basically sought to arrange the turbine of an exhaust-gas turbocharger as close as possible to the outlet openings of the cylinder in order thereby firstly to be able to make optimum use of the exhaust-gas enthalpy of the hot exhaust gases, which is determined significantly by the exhaust-gas pressure and the exhaust-gas temperature, and secondly to ensure a fast response behavior of the turbine and thus of the turbocharger. In this connection, it is therefore also fundamentally sought to minimize the thermal inertia and the volume of the line system between the outlet openings of the cylinders and of the turbine, which may be achieved by reducing the mass and the length of the exhaust lines.
The configuration of the exhaust-gas turbocharging often poses difficulties, wherein it is basically sought to obtain a noticeable performance increase in all rotational speed ranges.
According to previous systems, a severe torque drop is however observed in the event of a certain engine speed being undershot. Said torque drop is understandable if one takes into consideration that the charge pressure ratio is dependent on the turbine pressure ratio. For example, if the engine speed is reduced, this leads to a smaller exhaust-gas mass flow and therefore to a lower turbine pressure ratio. This has the result that, toward lower engine speeds, the charge pressure ratio and the charge pressure likewise decrease, which equates to a torque drop.
In the previous systems, it is sought to improve the torque characteristic of a supercharged internal combustion engine using various measures.
One such measure, for example, is a small design of the turbine cross section and simultaneous provision of an exhaust-gas blow-off facility. Such a turbine is also referred to as a wastegate turbine. If the exhaust-gas flow rate exceeds a critical value, a part of the exhaust-gas flow is, within the course of the so-called exhaust-gas blow-off, conducted via a bypass line past the turbine. This approach has the disadvantage that the supercharging behavior is inadequate at relatively high rotational speeds or in the case of relatively high exhaust-gas quantities.
The torque characteristic may also be advantageously influenced by multiple exhaust-gas turbochargers connected in series. By connecting two exhaust-gas turbochargers in series, of which one exhaust-gas turbocharger serves as a high-pressure stage and one exhaust-gas turbocharger serves as a low-pressure stage, the compressor characteristic map can advantageously be expanded, specifically both in the direction of smaller compressor flows and also in the direction of larger compressor flows.
In particular, with the exhaust-gas turbocharger which serves as a high-pressure stage, it is possible for the surge limit to be shifted in the direction of smaller compressor flows, as a result of which high charge pressure ratios can be obtained even with small compressor flows, which considerably improves the torque characteristic in the lower engine speed range. This is achieved by designing the high-pressure turbine for small exhaust-gas mass flows and by providing a bypass line by means of which, with increasing exhaust-gas mass flow, an increasing amount of exhaust gas is conducted past the high-pressure turbine. For this purpose, the bypass line branches off from the exhaust-gas discharge system upstream of the high-pressure turbine and opens into the exhaust-gas discharge system again upstream of the low-pressure turbine, wherein a shut-off element is arranged in the bypass line in order to control the exhaust-gas flow conducted past the high-pressure turbine. The response behavior of an internal combustion engine supercharged in this way is considerably improved in relation to a similar internal combustion engine with single-stage supercharging, because the relatively small high-pressure stage is less inert, that is to say the rotor of a smaller-dimensioned exhaust-gas turbocharger can be accelerated more rapidly.
The torque characteristic of a supercharged internal combustion engine may furthermore be improved by multiple turbochargers arranged in parallel, that is to say by multiple turbines of relatively small turbine cross section arranged in parallel, wherein turbines are activated successively with increasing exhaust-gas flow rate.
However, the inventors herein have recognized an issue with the above approaches. The rotational speed of an activatable turbine decreases drastically when the latter is deactivated, such that, upon reactivation, the rotor of said turbine may initially be accelerated in order to be able to generate and provide the desired charge pressure at the compressor side. The response behavior is thus impaired.
If the deactivated turbine is fed with a low exhaust-gas flow rate, the rotational speed of the deactivated turbine falls to a lesser extent, and a minimum rotational speed of the charger shaft can be ensured or maintained. The latter has a further relevant advantage. Specifically, if the rotational speed of the charger shaft falls below a minimum rotational speed, or if the charger shaft even comes to a standstill, the seal of the bearing arrangement of the oil-lubricated charger shaft can leak at the compressor side. Oil leakage at the intake side has severe disadvantages. If oil passes via the associated compressor into the intake system, the oil-contaminated fresh charge supplied to the cylinders adversely affects the combustion process, whereby, in particular, the untreated particle emissions can greatly increase. The oil may also be deposited on the inner walls of the intake system and impair the flow conditions in the intake system and/or in the compressor, and contaminate a charge-air cooler arranged downstream.
There are numerous reasons for the oil leakage. Firstly, the labyrinth seal that is generally used in the case of charger shafts appears to provide a satisfactory sealing action only when the charge shaft is at a certain minimum rotational speed. Secondly, when the turbine is deactivated, or when the compressor is not being driven, it is generally the case that a negative pressure prevails at the compressor-side end of the bearing arrangement, which negative pressure sucks or pulls the oil out of the bearing and into the intake system downstream of the non-driven compressor. In this context, it may be taken into consideration that, when the turbine is deactivated, the non-driven compressor is generally separated from the common intake system, wherein a blow-off line is preferably provided, which serves as a bypass line and which branches off from the intake system downstream of the compressor and opens into the intake system upstream of an additional operational compressor. An intake system of said type supports the generation of a negative pressure at the compressor-side end of the bearing arrangement when the turbine is deactivated, or when the compressor is not being driven. It is commonly the case that material bushings or rings, preferably rings with an open joint, are provided, that is to say arranged, in the labyrinth seal.
Accordingly, a method for operating a supercharged internal combustion engine having at least one cylinder head, having at least two cylinders and having an intake system for the supply of charge air to the at least two cylinders, in which each cylinder has at least two outlet openings for the discharge of the exhaust gases, at least one of which is in the form of an activatable outlet opening, each outlet opening being adjoined by an exhaust line for the discharge of the exhaust gases via an exhaust-gas discharge system, at least two exhaust-gas turbochargers are provided, each exhaust-gas turbocharger comprising a turbine and a compressor which are arranged on the same shaft, which shaft is mounted rotatably in an oil-lubricated bearing arrangement, the compressors of the at least two exhaust-gas turbochargers are arranged in parallel in the intake system, each compressor being arranged in a separate intake line of the intake system, and the separate intake lines merging, downstream of the compressors, to form an overall intake line, a first shut-off element is arranged, downstream of the first compressor, in the associated intake line, the exhaust lines of the activatable outlet openings of the at least two cylinders merge, with the formation of a first exhaust manifold, to form a first overall exhaust line which is connected to the turbine of the first exhaust-gas turbocharger, and the exhaust lines of the other outlet openings of the at least two cylinders merge, with the formation of a second exhaust manifold, to form a second overall exhaust line which is connected to the turbine of the second exhaust-gas turbocharger, is provided. The method comprises, proceeding from a deactivated first turbine and deactivated outlet openings, increasing a pressure at a compressor-side end of the bearing arrangement of the shaft of the first exhaust-gas turbocharger using at least one auxiliary mechanism.
In the method according to the disclosure, the generation of a negative pressure at the compressor-side end of the bearing arrangement when the first turbine is deactivated, or when the first compressor is not being driven, is counteracted, and/or the pressure prevailing at the compressor-side end of the bearing arrangement is increased. For this purpose, auxiliary mechanisms are used by which the relevant pressure is increased.
The oil-lubricated bearing arrangement of the shaft of a charger is generally connected, via a return line, to the crankcase of the internal combustion engine, wherein ambient pressure or positive pressure prevails in the crankcase. In interaction with the negative pressure that generally prevails at the compressor-side end of the bearing arrangement when the first turbine is deactivated, or when the first compressor is not being driven, a pressure gradient is thus realized across the bearing seal, which pressure gradient gives rise to oil leakage and forces or drives oil out of the bearing arrangement into the intake system at the compressor side.
The auxiliary mechanism that is used according to the disclosure for increasing the pressure may in this case assume a wide variety of forms, wherein it is also possible for multiple auxiliary mechanisms to be provided which are used jointly, that is to say simultaneously, or alternatively, that is to say alternately and so as to complement one another.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.